It is known in the field of strain sensors to embed an optical fiber having reflective elements, such as Bragg gratings, within a structure, as is described in U.S. Pat. Nos. 4,806,012, entitled "Distributed, Spatially Resolving Optical Fiber Strain Gauge" and No. 4,761,073, entitled "Distributed, Spatially Resolving Optical Fiber Strain Gauge" both to Meltz et al. The fiber containing gratings is also called a "fiber sensor." It is also known that optical fiber sensors can be embedded in composite materials or structures, thereby allowing strain and/or temperature measurement of the structure.
Typically, a broad wavelength band source light is launched into the fiber and the gratings reflect only a narrow wavelength band of the source light. The reflection wavelength of the gratings (i.e., the wavelength where a local maximum in reflection occurs), shifts as a function of both strain and temperature variation at the location of the gratings. Consequently, in order to determine strain independent of temperature variations, such a sensor requires a separate temperature measurement device (e.go, another grating). Thus, a single grating (or sensor) in such a configuration cannot provide temperature compensated strain measurement, nor strain compensated temperature measurement. Accordingly, the same inability to perform temperature and strain measurements from a single sensor exists for sensors embedded in structures.
Therefore, it would be desirable to provide an embedded optical fiber sensor which provides optical signals from a single grating from which both temperature and strain can be determined.